GaAs-AlAs Quantum Wells Research Articles

Stability of electron-hole plasma in type-I and type-II GaAs-GaAlAs single quantum wells

We calculate and compare energies of the electron-hole plasmas and ground-state excitons in type-I and type-II GaAs-GaAlAs single quantum wells with various structural conditions. Calculations are performed with a flexible and numerically stable method using realistic material parameters. We confirm that the exciton states are stable in type-I quantum wells for well widths up to 250 \AA{}, as described in previous work [G. E. W. Bauer and T. Ando, Phys. Rev. B 34, 1386 (1986)]. We find that the electron-hole plasma becomes stable, particularly in narrow type-II quantum wells, e.g., about 2.5 meV below the ground state exciton in GaAs-AlAs quantum wells whose widths are 6 and 4 monolayers, respectively. Moreover, calculations including electric fields reveal two different conditions of the stable electron-hole plasma: one is that electrons and holes are separated and the other is that they are confined in the narrow regions. The results suggest the possibility of observing the electron-hole plasma as the lowest excited state in the GaAs-GaAlAs quantum well.

Modulation of Fano Resonance by a Perpendicular Electric Field in GaAs-AlAs Quantum Well

We study the formation of Fano resonance in excitonic states in GaAs–AlAs quantum wells. Our calculation indicates that an electric field applied perpendicularly to the layers modulates the Fano resonance. We have found that for a given excitonic state there exists a field-range for which Fano resonance is well discernible. The electric field effects are more pronounced for thick wells.

Exciton localization and interisland exciton diffusion in interrupted-growth GaAs–AlAs quantum well

We present a study of the exciton localization and the interisland exciton diffusion in interrupted-growth thin GaAs–AlAs quantum wells (QW), by photoluminescence (PL). These quantum wells include extended monolayer flat regions. We have shown that, for a given position on the sample, the exciton localization is stronger in the wide well regions than in the narrow well regions. We have also developed a quantitative model, of independently distributed cylindrical holes at the AlAs/GaAs interface, to explain the evolution of the interisland exciton diffusion as a function of temperature and have obtained the average value of the narrow regions extension and the ratio of the area of the wide regions to the area of the narrow regions.

Magnetic field induced Mott transition of a one-component electron plasma in GaAs quantum wells

We have measured the time-resolved magneto-photoluminescence of mixed type GaAs AlAs quantum wells, in which the electron density can be varied from ∼10 9 to ∼10 11 cm −2. We confirm that as the electron density is increased the precursor of the Mott transition is the trion (negatively charged exciton) and that the transition occurs when the Landau level filling factor v = 2. In zero field the transition is broad and ill-defined, but in a field it is sharp and is accompanied by an abrupt change in linewidth and radiative lifetime.

High quality interfaces in GaAs–AlAs quantum wells determined from high resolution photoluminescence

High resolution photoluminescence (PL) measurements performed on several GaAs–AlAs quantum well structures revealed sharp excitonic transitions separated in energies corresponding to roughly half-monolayer fluctuations in well size. The narrow linewidths correlate with interface island structure whose lateral extent is either much larger or much smaller than the exciton diameter. The half-monolayer separation results from a sharply peaked PL intensity response occurring around those areas of the laterally nonuniform interface which have roughly 50% island coverage, with the average island size much smaller than the exciton diameter, about 225 Å.

Γ- X hybridization effects on the carrier subband transition times in type I GaAsAlAs quantum wells

We calculate the Γ → Γ carrier scattering times due to emission of confined and interface optic phonons taking into account the Γ- X hybridization of the electron states for type I GaAs/AlAs quantum well structures. We show that the intersubband transition times increase for well widths where the second GaAs electron subband becomes close in energy to the X point of the AlAs barrier and that this increase is strongly dependent on the amount of hybridization. These results suggests that the hybridization may affect carrier transport and relaxation and that intersubband scattering times could be used for measuring the hybridization of the second electron subband.

Spin splitting of excitons in GaAs quantum wells at zero magnetic field

We have observed an energy splitting between +1 and −1 excitons in GaAs AlAs quantum wells at zero magnetic field. The studies, which have been done by picosecond time-resolved photoluminescence spectroscopy, show that exciton spin splitting depends strongly on excitonic density and on the dynamics of spin relaxation: the spin splitting increases, raising the initial carrier density, and the time evolution of the spin splitting follows the exponential decay of the degree of polarization of the photoluminescence. For large carrier densities and short time delays the observed exciton spin splitting is as large as 4 meV. We present theoretical results which support the interpretation of experimental spin splittings in terms of inter-excitonic exchange interactions.

Finite Element Solutions to GaAs-AlAs Quantum Wells with Connection Matrices at Heterojunctions

In this paper, the irreducible finite element formulations for the solutions to the single-band effective-mass equation are developed to incorporate the connection conditions at the heterojunctions. The exact interface formulation utilizes the exact form of the connection matrix. The finite-difference interface formulation uses the finite difference approximation to the connection matrix. For the investigation of the GaAs-AlAs quantum well, the finite-difference interface formulation has to be employed, and the relative errors are shown to be less than 103. Then, this finite element formulation is utilized to compare the effects of the electric field on the GaAs-AlAs quantum well with the heterojunctions modeled by the conventional, the effective-mass-dependent, and the energy-dependent connection matrices. It is found that the eigenenergies predicted by the models with non-conventional connection matrices are larger in magnitude and less dependent on the electric field than the conventional model.

Theoretical analysis of resonant magnetotunneling spectroscopy of holes

We calculate the hole energy band structure in GaAs-AlAs Quantum Wells (QWs) in high in-plane magnetic fields and simulate Resonant Magnetotunneling Spectroscopy (RMS) experiments, recently proposed as a probe of the hole energy dispersion. We find that the high magnetic fields used in RMS result in small modifications of the QW hole subbands in thin QWs. However, for wide QWs or very large magnetic fields, the field strongly affects the hole dispersion by changing the heavy hole-light hole mixing. We give a simple expression to evaluate to which extent the mixing is modified as a function of the QW width and field strength.

Fano resonances in the excitation spectra of semiconductor quantum wells.

Fano resonances are observed in the photoluminescence excitation spectra of GaAs-AlAs quantum wells. They originate from the Coulomb interaction that couples a discrete state and a continuum of excitonic states belonging to different pairs of conduction and valence subbands. This interpretation is supported by our calculations and by the behavior of the excitonic line shape in the presence of a magnetic field applied perpendicular to the layers.

Lifetime of excitons in GaAs quantum wells

Time resolved luminescence measurements under resonant excitation were performed at 10 K on the free exciton in very high quality GaAs-AlAs quantum wells. At resonance, the luminescence intensity is increased by two orders of magnitude, and the decay is short. The radiative lifetime of the excitons in the radiant states is about 18 ps in agreement with the theory of Hanamura and Andreani

Role of interface optical phonons in cooling hot carriers in GaAs-AlAs quantum wells.

We present an experimental study of the nonequilibrium phonon population generated by the relaxation of hot carriers photoexcited in GaAs-AlAs quantum wells. The optical phonon population is measured by anti-Stokes Raman scattering performed in the plane of the layers. The measured occupation numbers suggest that interface phonon modes contribute significantly to the relaxation dynamics over the lifetime of an electron-hole pair. These results are explained on the basis of the interplay between emission and reabsorption of hot phonons, which leads to a large population of interface phonons under steady-state conditions.

A Study on Electroabsorption Effects in Coupled Quantum Wells

Field-induced effects of optical absorption in coupled quantum wells, comprising two GaAs-AlAs quantum wells separated by a thin AlAs barrier, have been studied. The coupled system evolves into an uncoupled one, leading to large variations of the absorption coefficient at the absorption edge through the field-induced decoupling effect. The large linear shift of the quantum energy levels and an effective blue shift of the absorption edge with electric field lead to unique polarization and wavelength dependence of the electroabsorption effect.

The effect of electric field on intrasubband and intersubband transitions via interface phonons in GaAs-AlAs quantum wells

The author calculates the scattering rates for intrasubband and intersubband transitions in GaAs-AlAs quantum wells due to interface phonons with an applied longitudinal electric field. The electron-interface-phonon (Frohlich) Hamiltonian used is that obtained from the Fuchs-Kliewer slab model, and the electron envelope wavefunction under the influence of an electric field parallel to the growth direction is obtained by a variational method. The usual selection rules for these transitions break down and the scattering rates are found to increase significantly when an electric field is applied. These scattering rates may even become the dominant scattering mechanism for large quantum wells and sufficiently high fields. He observes also that this change in scattering rates has an important dependence on the interface-phonon dispersion.

Electric-field induced ultrafast intersubrand transitions in large GaAs-AlAs quantum wells via electron-interface-phonon interaction

We obtain the scattering times for intersubband transitions in GaAs-AlAs quantum wells under the influence of an electric field (parallel to the growth direction). The electron-interface-phonon (Fröhlich) interaction is taken into account and scattering times faster than 100 fs are obtained for quantum well widths between 150 Å and 160 Å.

Strong anisotropy of hole subbands in (311) GaAs-AlAs quantum wells.

Strong anisotropy features (e.g., saddle points) displayed by excited states of (311) valence-band GaAs-AlAs quantum wells are determined through the exact solution of the effective-mass equation including the spin-orbit interaction. Qualitative changes in the topology of the subband energy surfaces occur as the well width is varied. The relevance of the present calculations for the interpretation of recent resonant magnetotunneling experiments on p-type (311) A double-barrier heterostructures is discussed

Atomic-scale characterization and control of semiconductor interfaces grown by molecular beam epitaxy: interface roughness and optical and electronic properties

We present current understandings on the atomic-scale structures of the GaAsAlAs and GaAsAlGaAs heterointerfaces grown by molecular beam epitaxy (MBE). The analysis of the photoluminescence (PL) and electron mobility of GaAsAlAs quantum wells (QWs) reveals that conventionally grown top (AlAsonGaAs) interfaces have monotomic steps with the lateral size L step of about 200 Å, whereas the roughness with similar step height and the smaller lateral size L step = 40−70 A ̊ is found at bottom (GaAsonGaAs) interfaces. Growth interruption is very effective to enhance the surface diffusion of gallium adatoms and to increase L step at the top interfaces, leading to the drastic reduction in PL broadening. By using the alternate-beam MBE technique and superlattice buffer layer below the QW, one can enhance L step at the bottom interface to some extent, resulting in the significant suppression of the scattering by interface roughness. Characterization by cross-sectional transmission electron microscopy supports our model of microscopic interface structures.

Binding energies of excitons in type-II GaAs-AlAs quantum-well structures in the presence of a magnetic field.

We report a calculation of the binding energies of both the heavy-hole and the light-hole excitons in type-II GaAs-AlAs quantum wells as a function of the size of the AlAs layer (or the GaAs layer) in the presence of a magnetic field applied parallel to the direction of growth. We follow a variational approach and assume that the electrons and holes are confined in infinite potential barriers. For a given set of values of GaAs- and AlAs-layer thicknesses, the binding energies of excitons are found to increase with increasing magnetic field.

Effect of magnetic fields on exciton binding energies in type-II GaAs-AlAs quantum-well structures.

We calculate the binding energies of both the light-hole and heavy-hole excitons in type-II GaAs-AlAs quantum wells in the presence of a magnetic field applied parallel to the axis of growth. Two methods are applied and compared: a variational approach and a perturbation method. The exciton binding energies are calculated assuming infinite potential barriers. The behavior of the exciton binding energies as functions of well widths and the magnetic field is discussed. For a given value of the magnetic field, the exciton binding energies are found to be larger than the zero-magnetic-field case. Results obtained from both methods are compared.

Field induced optical effects in coupled quantum wells

The electro-optical effects of GaAs-AlAs quantum wells coupled via a thin AlAs separating layer are studied theoretically and experimentally by absorption current spectroscopy. It has been found that such coupled quantum well structures exhibit unique absorption features that are different from isolated quantum wells, and large changes of the absorption coefficient are possible through the field induced decoupling effect involving both quantum well structures for electrons and holes. Application of the coupled quantum well structures in intensity modulators is also discussed. >